1. Field of the Invention
The present invention relates to a method for forming a Group-III nitride semiconductor layer and a Group-III nitride semiconductor device.
All of patents, patent applications, patent publications, scientific articles and the like, which will hereinafter be cited or identified in the present application, will, hereby, be incorporated by references in their entirety in order to describe more fully the state of the art, to which the present invention pertains.
2. Description of the Related Art
Group-III nitride semiconductors have relatively large forbidden band gaps and allow a direct transition between conduction and valence bands, for which reason applications of the Group-III nitride semiconductors to short wavelength light emitting devices have been on the aggressive investigations. Also, the Group-III nitride semiconductors have relatively large saturated drift velocities of electron and also allow a hetero-junction structure to generate a two-dimension carrier gas. These desirable properties will be attractive for applications to various electron devices.
The Group-III nitride semiconductors are, however, not suitable in micro-processability or etching-controllability, while improvement or increase in micro-processability or etching-controllability of the Group-III nitride semiconductors are essential for realizing desirable high performances of the Group-III nitride semiconductor device. In this circumstances, the importance for improving the controllability of micro-process or micro-etching has been on the increase.
The above technical issues, which are required to be solved, will hereinafter be described in detail by taking some typical examples of optical devices such as semiconductor lasers.
One typical example of the Group-III nitride semiconductors is gallium nitride. The Group-III nitride semiconductors are attractive for a light emitting diode or a laser diode for emitting a blue color laser. The laser diode is attractive as a light source for a large capacitive optical disk. In recent years, aggressive development of high output laser diodes as write-purpose light source has been made. For application to the optical disk, a fine or highly-controlled bean spot is essential, wherein it is important to control a transverse-mode. For high output, it is important to increase an efficiency of carrier injection. Further, high frequency performances are also important as transfer speed of the optical disk is increased. In order to improve the high frequency performances, possible reductions in not only resistance but also parasitic capacitance of the device are important.
FIG. 1 is a fragmentary schematic cross sectional elevation view of a typical example of a conventional structure of a nitride semiconductor laser diode with a ridge-structure for current confinement. The conventional structure includes an n-GaN substrate 501, an n-AlGaN cladding layer 502 over the n-GaN substrate 501, an n-GaN guide layer 503 over the n-AlGaN cladding layer 502, an InGaN multiple quantum well structure 504 over the n-GaN guide layer 503, a p-GaN guide layer 505 over the InGaN multiple quantum well structure 504, and a p-AlGaN cladding layer 506 over the p-GaN guide layer 505. The p-AlGaN cladding layer 506 has a ridge structure 508 which may be formed by an isotropic etching. A p-GaN layer 507 is provided over a top of the ridge structure 508. An insulating layer 510 is provided on the p-AlGaN cladding layer 506, wherein the insulating layer 510 has a stripe-shaped opening over the p-GaN layer over the top of the ridge structure 508. A p-electrode 509 is provided, which extends on the p-GaN layer and the insulating layer 510. The ridge structure 508 causes a current confinement. The control of the transverse mode may be made by adjustments in width and height of the ridge structure 508. The ridge-structured laser diode is advantageous or superior in high frequency performance due to its low parasitic capacitance.
On the other hand, another type laser diode with buried-current-confinement layer was proposed as realizing a higher current confinement efficiency than the above ridge-structured laser diode. Japanese laid-open patent publication No. 10-093192 discloses the buried-structure laser diode. FIG. 2 is a fragmentary schematic cross sectional elevation view of a typical example of a conventional structure of a nitride semiconductor laser diode with a buried current confinement structure.
The other conventional device includes an n-GaN substrate 401, an n-AlGaN cladding layer 402 over the n-GaN substrate 401, an n-GaN guide layer 403 over the n-AlGaN cladding layer 402, an active layer 404 over the n-GaN guide layer 403, a p-GaN guide layer 405 over the active layer 404, and a p-AlGaN cladding layer 406 over the p-GaN guide layer 405. Further, a buried current confinement layer 407 with a stripe-shaped opening is provided over the p-AlGaN cladding layer 406. The buried current confinement layer 407 may comprise GaN or AlN. A part of a top surface of the p-AlGaN cladding layer 406 is exposed through the stripe-shaped opening of the buried current confinement layer 407. A p-GaN contact layer 408 is provided over the buried current confinement layer 407 and within the stripe-shaped opening. A p-electrode 409 is provided on the p-GaN contact layer 408. A carrier injection is made through the stripe-shaped opening with a current confinement by the buried current confinement layer 407. The buried current confinement layer 407 improves a carrier injection efficiency.
Japanese laid-open patent publication No. 2001-15860 discloses still another conventional structure comprising an AlN buried current confinement layer with an stripe-shaped opening for current confinement and transverse mode control, wherein the AlN buried current confinement layer is provided either in a cladding layer or between the cladding layer and a light emitting layer.
The above two conventional techniques utilize the buried current confinement layer with the opening allowing the carrier injection with current confinement. The transverse mode depends upon respective thicknesses of the layers, which are controllable in growth processes. For these reasons, the laser diode with the buried current confinement layer with the opening would be more advantageous in reproductively and yield than the above-described ridge-structured laser diode.
The above ridge-structure shown in FIG. 1 may be formed by a lithography technique and a subsequent isotropic etching technique. It should be noted that a chemical etching to the nitride semiconductors is not available due to property of the nitride semiconductors, while a halogen-based dry etching to the nitride semiconductors is available. The transverse mode characteristic of the ridge structure depends upon p-electrode stripe width, ridge with and ridge depth as main parameters. The controllability or accuracy in the p-electrode stripe width and the ridge width depends on the accuracy of the lithography technique. On the other hand, the controllability or accuracy in the ridge depth depends on the etching-controllability which further depends on various parameters, for example, plasma conditions, etching gas flow rate, and substrate temperature in etching process. For those reasons, it is difficult to realize a high yield of the devices over a large area. Further, charge particles generated in the etching process may provide a damage to the active layer of the device.
With respect to the laser diode including the buried current confinement layer with the opening as shown in FIG. 2, if the current confinement layer comprises an n-GaN layer or an n-AlGaN layer, then a p-n junction is formed between the n-type current confinement layer and the p-type cladding layer or the p-type contact layer. This p-n junction causes a junction capacitance which may further cause a deterioration in high frequency performance. In order to avoid this problem, it is effective that the buried current confinement layer comprises an undoped nitride semiconductor, for example, undoped GaN or undoped AlGaN. The undoped nitride semiconductor is, however, higher in resistivity than the doped nitride semiconductor. Further, the undoped AlGaN or the undoped GaN grown over the n-type nitride semiconductor layer is likely to have an n-type conductivity. This means a difficulty of the crystal growth process for growing the undoped nitride semiconductor layer over the n-doped nitride semiconductor layer.
It should also be noted that the use of a single crystal AlN to the current confinement layer may improve the high frequency performance, but does provide the following problems or issues.
The first issue is concerned with a possible formation of crack which may be formed by differences in both lattice constant and thermal expansion coefficient between AlN and other nitride semiconductors such as AlGaN, GaN and InGaN. The other nitride semiconductors such as AlGaN, GaN and InGaN may be used for the cladding layers, the optical guide layers and the contact layer in the laser diode. In connection with the formation of the laser diode structure shown in FIG. 2, an undesired crack or cracks may be formed in three types processes. The first type process is an AlN deposition. The second type process is the deposition of GaN, AlGaN or InGaN over AlN. The third type process is substrate temperature rising or dropping.
The crack in connection with the first or second type process is due to the difference in lattice constant, and thus may be caused when a thickness of the AlN layer over the GaN, AlGaN or InGaN layer exceeds a critical thickness depending on the difference in lattice constant or when a thickness of the GaN, AlGaN or InGaN layer over the AlN layer also exceeds the critical thickness. In contrast, the crack in connection with the third type process is caused by variation or change in lattice constant due to a difference in thermal expansion coefficient between AlN and either GaN, AlGaN or InGaN. Even if no crack is caused in the AlN layer, it is highly possible that a crack or clacks may be caused in a top cladding layer over the crack-free AlN layer in the above second or third type process. Accordingly, it is difficult to suppress cracks completely. The crack in the AlN layer not only does render the current confinement layer dysfunctional in current confinement but also does cause the laser diode chip to be broken.
The second issue with the AlN current confinement layer is concerned with a difficulty in selective removal of the AlN current confinement layer. The above-described structures of FIGS. 1 and 2 need a selective removal of the AlN current confinement layer and a re-growth process for the p-type contact layer and the p-type cladding layer. A chlorine-based dry etching process is generally used for etching the nitride materials. It is difficult to realize a desired selective etching of AlN and either GaN, AlGaN or GaN by utilizing the chlorine-based dry etching process because of physical sputtering effect in the dry etching process. It is also difficult for the chlorine-based dry etching process to suppress undesired variation in etching depth of the nitride material due to variation in etching conditions. Namely, it is difficult for the chlorine-based dry etching process to realize a desired high etching-controllability of the nitride material. Therefore, the chlorine-based dry etching process has a difficulty to achieve a desired high yield and is likely to cause a problem with an etching-damage.
Japanese laid-open patent publication No. 9-232680 discloses a selective etching of AlN with an alkali solution such as KOH.
It is also disclosed by M. S. Minsky in Applied Physics Letter 68 (1996) 1531-1533 that these alkali etchants etch not only AlN but also GaN with an imperfect etching-selectivity and further cause an undesired deterioration in after-etching-morphology of the etched portion. The deterioration in the morphology causes a deterioration in crystal quality or crystal imperfection of re-grown layer. This is the significant problem.
Japanese laid-open patent publication No. 2001-15860 discloses that a stripe-shaped mask of SiO2, which may be etched, is formed for subsequent deposition of AlN layer prior to a selective removal of the deposited AlN layer by a left-off method. As far as this method is concerned, the coverage of side walls of the mask by the AlN layer makes it difficult to carry out the intended left-off method. In order to avoid this problem, it is necessary to limit the thickness of the AlN layer so that the side walls of the mask are not covered by the deposited AlN layer. The limitation in the thickness of the deposited AlN layer provides a limitation to the desired increase in the withstand voltage of the device, and also allows an undesired increase in the leakage of current, resulting in an insufficient current confinement.
A further problem is that residual impurities of the lift-off mask may cause a deterioration in the device. Particularly if the mask material is silicon oxide, for example, SiO2 or SiOx, then it is difficult to completely remove residual Si, for which reason it is likely that a pile-up of Si is formed over a re-growth interface of the cladding layer. The pile-up of Si may cause a deterioration of electric characteristics of the device.
The above-described technical issues caused by the difficulty in processing or etching the Group-III nitride semiconductor layer would also be applicable to not only the laser diode but also any other electron devices utilizing the Group-III nitride semiconductor layer.
At present, a typical structure of the field effect transistor including GaN-based compound semiconductors is the planar type. Notwithstanding, in order to realize such a high level performance as required, it would be essential make not only an optimization of the materials for the multi-layer structure but also a structural modification of the nitride semiconductor layer such as a recess by subjecting the nitride semiconductor layer to an etching process.
In the above circumstances, the development of a novel technique for realizing a high processability or a high etching-controllability to the Group-III nitride compound semiconductor layer free from the above problems is desirable.